In high pressure die casting, the complexity of a part is limited generally to straight pull cores with 1.degree. draft. Exceptions to this generalization are collapsible metal cores, "loose pieces" (which are constrained metal pieces which are not normally retractable, such as the pinion gear cavity of a gearcase), and high integrity expandable cores. When it is practical to use collapsible cores or loose pieces to achieve the desired shape in the casting, this is most economical. When the desired shape becomes very complex, such as the air/fuel mixture ports of a two-cycle engine, the use of loose pieces or retractable cores are not practical, hence the need for high integrity expendable cores.
The core required in high pressure die casting must be able to withstand a metallostatic pressure of 20,000 psi, dynamic forces resulting from a metal front advancing at 60 in/sec and metal temperature of 1200.degree. F. After this has been achieved and the metal casting has been made with the expendable core, the expendable core has to be removed from the casting.
The most widely used expendable, high integrity core used in high pressure die casting is a fused salt core. It can withstand the dynamic and static forces of the die casting process, the temperature of the metal and yet is easly removed by dissolving the core with water. It has been the general practice to make this core by a die casting process with certain inherent limitations which include:
1. The strinkage of the salt upon cooling is significantly greater than the shrinkage of the metal die. This leads to cracking of the core.
2. Often the process is limited to having isolated heavy sections due to the nature of the die casting process. This leads to shrinkage cavities which may later collapse during the die casting of the metal around the expendable core.
Therefore, there has been a need to improve upon the process for making salt cores.